JPH08111411A - Manufacturing for ferroelectric thin film - Google Patents

Abstract

PURPOSE: To provide a ferroelectric thin film without including a material vaporized at a low temperature, by using a mixed material of alkoxide, carboxylate, acetyleacetonate or a combination of these materials in a layered oxide structure. CONSTITUTION: A mixture of alkoxide, carboxylate, acetyleacetonate or a combination of these materials in a layered oxide structure is used with a solvent to prepare a precursor solution. The precursor is applied to a face of a substrate 10 to form a thin film, and the substrate 10 is baked in a given time to remove the solvent from the thin film. The substrate 10 with the thin film is heated to form a crystalized thin-film oxide material 13 with a layered structure. Then, a ferroelectric thin film made of oxide material with a layer structure that doesn't include a material vaporized at low temperatures can be obtained. In a non-volatile memory, a fatigue problem, and a deterioration problem depended on time, such as dielectric breakdown or aging, can be prevented.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a ferroelectric layered structure oxide in the form of a thin film, and to a method for producing the same using chemical solution processing based on processes such as organometallic decomposition and sol-gel. To do. Such devices find application in ferroelectric devices such as capacitors, non-volatile memories, sensors, displays, and transducers.

[0002]

2. Description of the Related Art Ferroelectric substances are characterized by having spontaneous polarization, and the polarization direction can be reversed by an electric field. In addition, these materials have unique dielectric, pyroelectric properties, applied to a wide variety of devices such as capacitors, dielectric resonators, thermal sensors, transducers, actuators, non-volatile memory, optical waveguides, and displays. It exhibits piezoelectric and electro-optical properties. However, in order to apply to such various devices, it is necessary to manufacture a ferroelectric material in a thin film form, thereby utilizing the above various characteristics to increase the degree of freedom in designing such a thin film structure. Is effective.

Since it is required to achieve bulk properties of a ferroelectric material in a thin film form for application to various devices, stoichiometry, crystallinity, density, microstructure, crystal orientation, etc. It is necessary to use deposition techniques that can optimize the various properties exhibited by the thin film. Although various deposition techniques have been used to date, techniques for growing films with controlled various properties under relatively low temperature conditions are still in the research stage. Several techniques are currently being developed to achieve this. In general, thin film deposition techniques can be broadly classified into the following two categories. That is,
(1) Physical Vapor Deposition (PVD) and (2) Chemical Process ("The Materials Science of Thin Films", Milto
n Ohring, Academic Press, 1992; SL Swartz, IEEE T
ransactions on Electrical Insulation, 25 (5), 1990,
935; SB Krupanidhi, J. Vac. Sci. Technol. A, 10
(4), 1992, 1569). This chemical process is further divided into two subgroups. That is, chemical vapor deposition (CVD), and sol-gel and organometallic decomposition (Me
talorganic Decomposition: MOD) is a wet chemical process. The most commonly used ferroelectric thin film deposition method in PVD technology is electron beam evaporation,
rf diode sputtering, rf magnetron sputtering, dc magnetron sputtering, ion beam sputtering, molecular beam epitaxy, and laser ablation. While PVD technology offers a number of advantages in dry processes, including high purity and compatibility with semiconductor integrated circuit processes, these processes have the disadvantages of low throughput, slow deposition rate, and chemical It has the drawbacks that it is difficult to control stoichiometrically, the post-deposition annealing process must be carried out at high temperature, and the cost of the device is high. Further, according to the CVD technique, a film with excellent uniformity can be obtained, composition control is easy, both film density and deposition rate are high, step coverage is excellent, and a large-scale process is possible. There are various advantages of being. However, the selection of a suitable precursor is a very important issue. Moreover, the chemistries involved in this process (eg, organometallic CVD) can be quite complex, especially when accompanied by complex compositions unique to ferroelectric materials.

In recent years, chemical solution deposition methods including metalorganic decomposition (MOD) processes and sol-gel processes have received a great deal of attention as a method for producing thin films or coating films on a plurality of different substrates. The sol-gel process and the MOD process have become popular as thin film deposition techniques because of their simplicity. According to these processes, a film with a uniform molecular level on an atomic scale can be obtained in a multi-component system, high throughput can be achieved, and composition control can be performed as long as each different organometallic compound is dissolved in an ordinary appropriate organic solvent. It has the advantage that it is excellent, and the cost is low because the membrane can be manufactured at room temperature and pressure without the need for vacuum.

Ferroelectric devices currently being manufactured and developed in order to demonstrate their effects by a technique based on a chemical solution include pyroelectric detectors, ultrasonic sensors, surface acoustic wave devices, and Such electro-optical elements can be cited, but what has been the biggest stimulus to research activities of ferroelectric thin films in recent years is the great demand for commercialization of nonvolatile memories. As described above, the ferroelectric substance is characterized by having spontaneous polarization that can be reversed by reversing the applied electric field. The polarization in this material is
The applied electric field exhibits hysteresis. That is, at zero electric field, there are two stable equivalent polarization states, + P _r or -P _r, as shown in FIG. With such characteristics, an electrically invertible bistable element having a ferroelectric capacitor (metal-ferroelectric-metal) structure is realized. It is possible to encode either of these two states as "1" or "0" in the computer memory, and it is necessary to apply an external electric field (power) to maintain this state of the device. Therefore, this element can be regarded as a non-volatile memory element. To switch this state of the device, a threshold electric field (coercive electric field) exceeding + E _c or −E _c is required. In order to reduce the required applied voltage, the ferroelectric substance must be thinned. Non-volatile random access memory devices can only be obtained by integrating ferroelectric thin film capacitors into current VLSI (JF Scot
Science by T. and CA Paz de Araujo, 246, (198
9), 1400-1405). In addition to non-volatility, ferroelectric random access memory (FRAM) has various advantages such as fast switching speed, low operating voltage (less than 5V), wide operating temperature range, and excellent radiation resistance. Have. Furthermore, the ferroelectric thin film, the electrodes, and the passivation layer can be deposited in a separately located small device, thus eliminating the need to make any changes to the current on-line Si or GaAs VLSI fabrication. . In principle, FRAM is static RAM (SRAM) in cache memory and dynamic RAM (DRA) in main system memory.
M) and EEPROM (electr in the lookup table)
It could eventually be used instead of icalerasable programmable read only memories).

[0006]

Although there is a great potential for non-volatile RAM by using a ferroelectric thin film, ones that hinder commercialization adversely affect the life of the ferroelectric device. Deterioration problems such as fatigue, leakage current, and aging are serious. A common cause of deterioration in the properties of ferroelectric oxides is the presence of defects such as oxygen vacancies in the material. Regarding the problem of fatigue, it should be noted that the polarizability of a ferroelectric decreases as it is repeatedly inverted. Causes of fatigue (IK Yoo and SB Desu, Mat. S
ci. and Eng., B13, (1992), 319; IKYoo and SB
Desu, Phys. Stat. Sol., A133, (1992), 565; IK Yo
o and SB Desu, J. Int. Mat. Sys., 4, (1993), 49.
0; SB Desu and IK Yoo, J. Electrochem. Soc., 1
40, (1993), L133), the relative movement of oxygen vacancies,
And both are trapped at the electrode / ferroelectric interface (and / or grain boundaries and domain boundaries). These defects (with the desired ferroelectric phase)
It occurs during processing of the ferroelectric film and can be classified into two types: intrinsic defects and extrinsic defects. What is an extrinsic defect?
Impurities that are mixed into the film during processing and can be controlled by controlling the process environment.
Intrinsic defects can be classified into two types: That is, (a) a defect that maintains a stoichiometric ratio such as a Schottky defect, and (b) a defect that changes the stoichiometric ratio in a substance. As an example of the formation of such defects, Pb which is a ferroelectric thin film material which has been most extensively studied for application to a non-volatile memory.
An example is the case where Zr _x Ti _1-x O ₃ (PZT) is used. A Schottky defect in a perovskite (ABO ₃ ) ferroelectric substance such as PZT is represented by the following quasi-chemical reaction formula (in Kroger-Vink notation).

[0007] _{A A + B B + 3O O} → V A "+ V B '''' + 3V O ∞ + A S + B S + 3O S (1) where, _{A A, B} B, and _O O is, A, respectively, B, and Represents a site occupied by O, V _A ″, V _B ″ ″ and V _O ∞ represent vacancies of A, B, and O atoms, and A _S , B _S , and O _S respectively. Represents a Schottky defect. A typical example of a defect that changes the stoichiometric ratio is a vacancy formed by vaporization of one or more volatile constituent elements in a multi-component oxide material. In the case of PZT, for example, a treatment at a temperature of at least 600 ° C. is necessary to form the ferroelectric perovskite phase. However, the PbO component begins to vaporize as early as 550 ° C., so that oxygen vacancies and lead vacancies are formed as shown in the following formula.

[0008] _{Pb Pb + Ti Ti + Zr Zr} + 3O O → xPbO + xV Pb "+ xV O ∞ + (1-x) Pb Pb + Ti Ti + Zr Zr + (1-x) O O (2) intrinsic defects also ferroelectric It is also generated by the stresses that occur in the film during domain switching, the relative migration of these oxygen vacancies and their vacancy to the electrode / ferroelectric interface (and / or grain boundaries and domain boundaries). It has been quantitatively shown that pore traps are an important factor in the degradation of ferroelectric oxides (SB Des
u. and IK Yoo, J. Electrochem. Soc., 140, (1
993), L133). In order to explain this point, the case where fatigue occurs can be referred to. As described above, fatigue in a ferroelectric thin film is a decrease in polarization that occurs as the number of times the polarization is inverted increases. Under an externally applied ac electric field (which is necessary to generate the reversal of polarization), oxygen vacancies tend to move toward the interface as a result of instability at the electrode / ferroelectric interface. There is. Eventually, these defects get trapped at the interface,
Causes structural damage. This results in a decrease in polarization in the substance.

There are two possible solutions to overcome the problems of fatigue and other degradations. The first solution is
By changing the properties of the electrode / ferroelectric interface,
To reduce the tendency to trap. Multilayer electrode structures with ceramic electrodes such as RuO ₂ that can minimize trapping of oxygen vacancies have been used to minimize fatigue problems in ferroelectric oxides (US Patent Application No. 08 / 104,861 (Multilayer E
lectrodes for Ferroelectric Devices, filed August 10, 1993)). The second solution involves controlling the defect density. The extrinsic point defect concentration can be minimized by lowering the impurity concentration or compensating for the impurities. La and Nb doping is known as a technique to reduce the fatigue rate of PZT thin films on Pt electrodes by compensating for vacancies (SB Desu, DP.
Mat. Res. Soc. Symp., 335 by Vijay and IK Yoo
(1994), 53). Measures to minimize the concentration of intrinsic defects include, for example, selection of compounds having essentially high defect formation energy, and selection of compounds having no volatile component in the ferroelectric sublattice. Can be mentioned. Therefore, as another measure to overcome the problems of fatigue and other deteriorations, use of a ferroelectric compound containing no volatile component in the ferroelectric sublattice can be mentioned. A large number of layered structure ferroelectric oxides satisfying such adoption criteria are known.

In the past, layered structure oxides were not seriously considered as candidates for application in ferroelectric devices. However, the layered structural material Bi ₄ Ti ₃ O ₁₂
Attempts have been made to use as a gate material on transistors applied to switching memories (IE by SYWu
EE Transactions on Electron Devices, August 1974, 49
See pages 9-504). However, the device deteriorated quickly, and was not suitable for memory applications (SY Wu, Ferroelectrics, 1976, Vol. 11, 379-3).
See page 83). It is believed that attempts to develop practical devices using layered structure oxides have been hindered by the inability to deposit high quality thin films of such materials. There is.

The present invention has been made in view of the above problems, and an object thereof is to provide a highly reliable deposition process for producing a ferroelectric thin film using a layered structure oxide.

[0012]

A method of manufacturing a ferroelectric thin film according to the present invention is a method of depositing a thin film of a ferroelectric layered structure oxide on a substrate by chemical solution deposition,
Preparing a precursor solution containing a mixture of metal elements of a layered structure oxide in the form of alkoxide, carboxylate, or acetylacetonate, or a combination thereof, and preparing the precursor solution on the substrate A step of forming a thin film on the substrate, a step of baking the substrate having the film for a certain period of time to remove the solvent from the film, and a step of heating the substrate having the film to form a crystalline thin film layer. Forming a structured oxide, whereby the above object is achieved.

The ferroelectric layered structure oxide is preferably at least one of the following compounds: A _n Bi ₃ Ti _{n + 1} RO _{3n + 9} ABi ₂ R ₂ O ₉ Bi _{2n + 2} Ti _{4-n O 12-n a} n + 1 Bi 4 Ti n + 4 O 15 + 3n , where, a is Ca, Pb, Sr, or Ba,, R
Is Nb or Ta, and n = 0 or 1.

In one embodiment, the ratio of Ta to Nb is approximately 0.4.

Also, in one embodiment, the substrate is
Semiconductor composed of at least one of Si, Si coated with SiO ₂ or GaAs, sapphire, Zr
O ₂ , MgO, SrTiO ₃ , BaTiO ₃ , or P
a single crystal insulator composed of at least one of bTiO ₃ and at least one of complex integrated circuits.

The substrate is a metal electrode containing Pt, Al, Au or Pd, MO _x (0 <x <2) (where M is Ru, Rh, Ir, Os, Re or LSC).
O (lanthanum-strontium-cobalt salt))
-Containing conductive oxide electrode, TiN or ZrN-containing conductive nitride electrode, and YBa ₂ Cu ₂ O _{7- x}
Alternatively, the substrate may be covered with a conductive substance containing at least one superconducting oxide containing Bi ₂ Sr ₂ Ca ₂ Cu ₃ O ₁₀ .

The substrate and the coating film may be separated by an adhesive layer.

[0018] In one embodiment, the preparation of the precursor includes a hydrolysis process and a condensation process.

In one embodiment, the solvent is
It is at least one of alcohol, aromatic hydrocarbon and ester.

[0020] In one embodiment, the precursor contains an excess amount of 1% to 80% of one or more metal elements of the layered structure oxide.

In one embodiment, a spin coating process, a dipping process, a spraying process,
Alternatively, the precursor is applied to the substrate using at least one of the coating processes.

Further, in one embodiment, the firing is
It is carried out at a temperature between 50 ° C and 250 ° C.

In one embodiment, the heating is
It is carried out at a temperature between 500 ° C and 850 ° C. Also, in some embodiments, the heating is performed in air or in a controlled atmosphere of at least one of oxygen, nitrogen, argon, or other inert gas.

[0024] In one embodiment, the fixed time is from 1 second to 20 hours.

In one embodiment, the layered structure material belongs to a solid solution of SrBi ₂ (Ta _x Nb _2-x ) O ₉ (0 <x <2), and the precursor is strontium-
2-ethyl hexanoate _{(Sr (C 7 H 15 COO} ) 2,
About 99.8%), 50% excess of bismuth-2-ethylhexanoate (Bi (C ₇ H ₁₅ COO) ₃ , about 99%),
Tantalum ethoxide (Ta (OC ₂ H ₅ ) ₅ , approximately 99.95
%), Niobium ethoxide (Nb (OC ₂ H ₅ ) ₅ , about 9
9.95%) and 2-ethylhexanoic acid (C ₇ H ₁₅ COO
H, approximately 99%), the solvent is xylene,
The substrate is Si / SiO ₂ / Ti / Pt, the step of applying the precursor to the substrate is a spin coating method, the firing temperature is 250 ° C., and the firing time is 3 minutes. And the heating temperature is 750 ° C. and the heating time is 1 hour.

In one embodiment, the fatigue rate of the ferroelectric thin film is slow and the amount of leakage current is small.

Further, an embodiment further includes a step of manufacturing a ferroelectric element, a piezoelectric element, a pyroelectric element, or an electro-optical element using the deposited film.

[0028] Also, an embodiment further includes a step of manufacturing a non-volatile memory device using the deposited film.

Further, an embodiment further includes a step of manufacturing a capacitor element using the deposited film.

[0030]

The above-described method for producing a ferroelectric thin film according to the present invention makes it possible to produce a ferroelectric thin film that does not contain a component that vaporizes at low temperatures. The ferroelectric obtained by the manufacturing method of the present invention is an oxide having a layered structure. Preferably, the material of the layered structure oxide is A _n Bi ₃ Ti _{n + 1.}
RO _{3n + 9} , ABi ₂ R ₂ O ₉ , Bi _{2n + 2} Ti _4-n O _12-n , A
_{n + 1} Bi ₄ Ti _{n + 4} O _{15 + 3n} , where A = Ca, P
b, Sr or Ba, R = Nb or Ta, and n = 0 or 1 (according to EC SubbaRao
J. Phys. Chem. Solids, 23 (1962) 665; B. Aurivilliu
Arkiv Kemi, 1 (1949) 463; EC Sabba Rao J. Chem. Phys., 34 (1961) 695; GA Smolenski,
Fiz Tverdo by VA Isupov and AI Agranovskaya
See Go Tela, 3 (1961) 895). These compounds have pseudo-square symmetry, and along the pseudo-square c-axis (Bi
_It has a structure in which units similar to perovskite are laminated between ₂ O ₂ ) ²⁺ layers. Many of these compounds contain no volatile components in their sublattice that exhibit spontaneous polarization.
The formation of defects such as oxygen vacancies and the resulting degradation problems such as fatigue can be minimized in this way.

The method for producing a ferroelectric thin film of the present invention is a process based on a chemical solution, and is a mixture of a metal alkoxide, a metal carboxylate or a metal acetylacetonate of each element of the target layered structure oxide and a suitable solvent. A step of synthesizing a precursor solution containing a solution, a step of applying these solutions to a substrate material by a technique such as a spin coating method, a dipping method, a spray method, or a coating method; Is included in the precursor and a step of performing annealing to obtain the target crystalline layered structure oxide, whereby a ferroelectric thin film having excellent film quality can be obtained.

The substrate material used in the present invention is preferably a Pt-coated silicon wafer (Pt / Ti / SiO ₂ /
Si), RuO _x coated silicon wafer (RuO _x / SiO
₂ / Si), sapphire, or MgO. Preferably, the metal coating in the device is Pt, MO _x (where M = Ru, Ir, Rh, Os, etc.), YBCO (yttrium-barium-copper oxide), LSCO (lanthanum-).
Strontium-cobalt salt), Au, Pd, Al or Ni. Preferably, the ratio of Ta to Nb is about 0.4. By using these materials, the characteristics and reliability of the ferroelectric element are improved.

[0033]

[Examples] Manufacturing methods using a chemical solution including a sol-gel method and a MOD method have attracted much attention as a method for manufacturing powders, spheres, and fibers. In recent years, this technique has been expanded and used for manufacturing thin films or coating films on different substrates. This method uses a ferroelectric thin film,
It is applied to thin films including high temperature superconductors, conductive films, optical films, and protective films. The chemical solution deposition process consists essentially of the steps of precursor solution preparation, hydrolysis, substrate deposition, solvent removal, pyrolysis, and film crystallization. In general, processes for thin film deposition by solution are divided into two categories: sol-gel process and MOD process. For the MOD process, no hydrolysis occurs before pyrolysis. This process only involves pyrolysis and annealing of the film, whereas the sol-gel process consists of hydrolysis, dehydration reactions, and final polymerization into a gel or film. Usually, the organometallic compound used in the MOD method has oxygen as a bridge between the metal and the organic ligand. Therefore, in the case of the MOD method, an MOM (metal-oxygen-metal) bond is formed during the thermal decomposition of the organometallic compound on the substrate, whereas in the sol-gel process, the MOM is formed in the gel. A bond is created. In the sol-gel process, the solution gels upon hydrolysis and condensation of the precursor. After the hydrolysis step, the polymerization further proceeds by a dehydration reaction, thus forming an MOM network. In the MOD process, deposition of a film from a precursor solution on a substrate is followed by solvent removal and thermal decomposition.

The deposition process can be accomplished by spin coating, dipping, spraying, or applying a precursor solution onto the substrate. The as-deposited wet film is pyrolyzed and converted to a dry, hard film with a considerable amount of shrinkage. During this pyrolysis, physical morphological variations such as thickness, cracking, surface roughness, residual stress and non-uniform nucleation are important. After removal of residual organics by decomposition or polycondensation reactions, the wet film is converted into a dry film composed of complex oxides (amorphous state). Further crystallization is achieved by a sintering mechanism in which the pores in the film collapse, thus obtaining a dense film.

In carrying out the above process, one of the most important points is to consider and select an appropriate precursor. Properties of precursors and solvents, their concentrations, pH
Value, and viscosity are all important factors affecting the quality of the deposited film. An ideal precursor used in a chemical solution deposition process should have the following properties. That is, (1) high metal content, (2) high solubility in an ordinary organic solvent, (3) stable at room temperature and normal pressure,
(4) Pyrolysis is possible without evaporation or melting, (5) The obtained film is not contaminated with the precursor organic matter, and (6) each precursor is also a multi-component oxide. They should be compatible with each other and (7) non-toxic. However, not all of these criteria can be met simultaneously. For example, as the chain length of the organic ligand increases, the solubility usually increases, but the metal content decreases. Therefore, when choosing a precursor, a compromise must be found in these requirements. Basically, precursors used in chemical solution processes fall into three distinct classes. That is, (1) metal alkoxide M
(OR) _x , (2) metal carboxylate M (OOC
R) _x and (3) metal acetylacetonate M (C ₅ H
₇ O ₂ ) _x . Here, M is a metal, R is an alkyl group, and x is the atomic value of the metal. Metal alkoxides are commonly used in sol-gel processes, while MO
The D process uses a carboxylate.

Processes using solution-based methods for the fabrication of ferroelectric thin films have just been recently developed (Swartz, SL, IEEE Transactions on Electrical In.
sulation, 25, 5, 1990). Much of the research on solution processing of ferroelectrics is based on PbTiO ₃ , (Pb, L
a) Perovskite ferroelectrics such as TiO ₃ and Pb (Zr, Ti) O ₃ have been made (KD Budd, SK
Dey and DA Payne, Br. Ceram. Proc., 36, 1985, 1
07; RA Lipeles, DJ Colemen, and MS Leung,
Mat. Res. Soc. Symp. Proc., 73, 1986, 665; G. Yi,
Z. Wu, and M. Sayer, J. Appl. Phys., 64 (5), 198.
8, 2717; SL Swartz, PJ Melling, and CSGran
t, Mat. Res. Soc. Symp. Proc., 152, 1989, 227-232;
And GH Haertling, J. Vac. Sci. Technol., A9
(3), 1991, 414). However, the production of the layered structure ferroelectric thin film by this method has not been successful. The present invention provides a method for manufacturing a ferroelectric layered structure oxide thin film. The effectiveness of the process of the present invention for producing a ferroelectric layered structure oxide film, a particular class of ferroelectric layered structure material _{(i.e., SrBi 2 Ta 2 O 9 -SrBi} 2 N
A specific ferroelectric element (ie, solid solution of b ₂ O ₉ ) (that is,
A particular embodiment of the invention (i.e., MO, described in connection with the fabrication of ferroelectric capacitors applied to non-volatile memories).
D and spin coat). It is emphasized that the attached drawings and the specific embodiments of the present invention shown in the specification are merely examples, and should not be construed as limiting the invention described in the claims. I want to.

FIG. 2 is a schematic diagram showing a ferroelectric capacitor in which the ferroelectric substance is a layered structure oxide. The ferroelectric capacitor is formed on the top surface of the substrate material 10, which can be silicon, a layer of silicon dioxide deposited on a silicon chip, gallium arsenide, MgO, sapphire, or the like. Of course, the substrate 10 is a silicon dioxide layer,
A multilayer structure having various circuit elements formed on a silicon chip having a polysilicon layer, an ion-implanted silicon layer, or the like can be formed, whereby a complicated integrated circuit can be formed. On the top surface of the substrate 10, a standard P
The thin bottom electrode layer 12 is deposited using either the VD process or the chemical process for thin film deposition described above.
The material of the lower electrode is a metal such as Pt, Au, Al or Pd, a conductive oxide such as MO _x (0 <x <2) (where M = Ru, Rh, Ir, Os or Re), Ti.
Conductive nitrides such as N and ZrN, or YBa ₂ C
A superconducting oxide such as u ₃ O _{7- x} and Bi ₂ Sr ₂ Ca ₂ Cu ₃ O ₁₀ can be used. If desired, an intermediate adhesive layer 11 can be provided to improve the degree of adhesion between the lower electrode 12 and the substrate material 10. For example, when depositing Pt on a Si / SiO ₂ substrate, a thin Ti intermediate layer is provided to increase the degree of adhesion between Pt and SiO ₂ . Thereafter, the ferroelectric substance 13 which is a layered structure oxide is deposited on the lower electrode 12 according to the process of the present invention described later. The material of the upper electrode 14 is then deposited through a shadow mask to form an electrode directly on a required area, or this material is once deposited on the entire surface of the ferroelectric film and appropriately masked. After application, some capacitors are formed on the wafer by etching using any of the standard VLSI etching processes such as reactive ion etching, wet etching, ion milling, plasma etching. The material of the upper electrode 14 may be the same as that used for the lower electrode 12 here, or may be a combination thereof. If necessary, the buffer layers 15 and 16 are connected to the ferroelectric layer 13 and the lower electrode 1 as shown in FIG.
2 and between the ferroelectric layer 13 and the upper electrode 14 may be additionally laminated.

In a particular embodiment of the invention, the organometallic decomposition (MOD) process is used to prepare the starting material and a spin coating process is used to produce Si / SiO ₂ / T.
The solution was applied on the i / Pt substrate. This substrate was chosen because it is popular for non-volatile random access memory. For this purpose, a SrBi ₂ (Ta _x Nb _2-x ) O ₉ solid solution with a composition of x = 0 to 2 was chosen as the layered structure oxide. As a precursor for producing a thin film composed of these compounds, strontium-2-ethylhexanoate (Sr (C ₇ H ₁₅ COO) ₂ , 99.8%), bismuth-2-ethylhexanoate (Bi (C ₇ H ₁₅ C
OO) ₃ , 99%), tantalum ethoxide (Ta (OC ₂ H
₅ ) ₅ , 99.95%), niobium ethoxide (Nb (OC
₂ H ₅ ) ₅ , 99.95%) and 2-ethylhexanoic acid (C ₇
H ₁₅ COOH, 99%) and xylene was used as the solvent. A flow chart for preparing these starting materials is shown in FIG.

The process was initiated by measuring the required weight of the precursor for each metal, based on the mole percent of metal in the final compound. In some cases, for example when using a bismuth precursor, an excess weight of precursor was added to compensate for vaporization of the bismuth oxide during pyrolysis. 2-ethylhexanoic acid is added to convert Ta ethoxide and Nb ethoxide to ethyl hexanoate,
The solubility of these precursors in strontium ethyl hexanoate and bismuth ethyl hexanoate was enhanced. The acid was added in sufficient amount in tantalum ethoxide and niobium ethoxide to complete the conversion. The weight / volume of solvent (xylene) added was determined according to the required concentration of final stage precursor. As the solvent, aromatic hydrocarbons such as xylene, alcohols and esters can be used. In the first step (see Figure 4), a predetermined amount of tantalum ethoxide and niobium ethoxide was added to a pre-measured amount of 2-ethylhexanoic acid.
The solution was stirred at 100-120 ° C. for 30 minutes to achieve the conversion of metal ethoxide to metal hexanoate. After cooling
A stoichiometric amount of strontium-2-ethylhexanoate was added and the solution was stirred at 125-140 ° C for 30 minutes. After this, the bismuth-2-ethylhexanoate precursor was added. As described later in this section, it was found that adding a 50% excess of the precursor provided the film with good ferroelectric properties. The resulting solution was stirred at 130-150 ° C for 10-20 hours to confirm complete miscibility of the above precursors.
The mixture was then filtered using a 0.45 μm filter to remove all types of particles that might be present as impurities in the solution. Xylene, which is a solvent, is added to the concentrated precursor solution, and the concentration of the solution is adjusted.
It was set to 0.1 mol / liter.

Then, a 0.1 mol / liter solution was spin-coated on a 3-inch Si / SiO ₂ / Ti / Pt wafer to prepare a thin film of these solid solutions. In this process, the film thickness formed for each coat is controlled by the concentration of the precursor, the rotation speed and the rotation time. The desired film thickness can also be obtained by repeating the spin coating / baking process several times. In this particular embodiment, 1500-2000 revolutions per minute (rp)
A small droplet of the precursor solution was dropped on the substrate rotated at the speed of m). The wafer was rotated at room temperature and atmospheric pressure for a total of 20 to 40 seconds. Then, this wafer was baked at 220 to 250 ° C. for 1 to 3 minutes to surely remove organic substances from these films. This spin coating / baking process was repeated several times to obtain films having different thicknesses. These film thicknesses were determined separately by scanning electron microscopy (SEM) and angle-variable spectroscopic ellipsometry (VASE). As an example, an SrBi ₂ Ta ₂ O ₉ (SBT) film and SrBi ₂ Nb ₂ obtained by performing various spin coating / firing cycles
The thickness of the O ₉ (SBN) film is shown in Tables 1 and 2. Then, the wafer containing these thin films was annealed in a conventional tube furnace at a temperature of 750 ° C. for 3 hours in an oxygen atmosphere. By this heat treatment, all kinds of solvents and constituent elements in the film were removed, and a dense film was produced.
This annealing step was performed for crystallizing the film. Crystallization in this process is achieved by a sintering mechanism in which the pores in the film collapse, resulting in a dense film. This heat treatment step may be carried out in air or in nitrogen, argon or other inert gas.

[0041]

[Table 1]

[0042]

[Table 2]

The microstructure, composition, and crystalline phase of the ferroelectric film are the decisive factors controlling the characteristics of the device. Therefore, the characteristics of the SrBi ₂ (Ta _x Nb _2−x ) O ₉ (SBTN) film deposited on the Si / SiO ₂ / Ti / Pt substrate are first determined by their optical characteristics, microstructure, composition and crystal phase, respectively. Ellipsometry, scanning electron microscope (SEM) method, x-ray photoelectron spectroscopy (XPS)
Method, energy dispersive spectroscopy (EDS) method, and x-ray diffraction (XRD) method. A variable angle spectroscopic ellipsometry method was used to determine the thickness and optical constants of the deposited films. Ellipsometry angles delta and psi were measured as a function of wavelength (250 nm to 1000 nm) at incident angles of 70 °, 75 ° and 80 °. The Cauchy dispersion relationship was estimated and used to calculate film thickness and film refractive index. Plots of refractive index and extinction coefficient as a function of wavelength for SBT and SBN films annealed at 750 ° C. for 3 hours are shown in FIGS. 5 and 6. The thickness of these films was determined to be around 200 nm. The refractive index obtained was close to the observed refractive index of the corresponding bulk compound, thus displaying a very good film packing density. This was further confirmed by observing the surface morphology of the film using scanning electron microscopy. 7 and 8 respectively show 750
1 shows a typical example of SEM micrographs obtained from SBT and SBN films annealed at 3 ° C. for 3 hours. These films showed a crack-free, dense, and molecular-level uniform microstructure. The average particle size of the SBT film and the SBN film annealed under the above conditions is about 0.2.
μm. The composition of these films was determined using the EDS method (bulk of the film) and the XPS method (surface of the film).
Table 3 shows the results obtained on the SBT film annealed at 750 ° C. for 3 hours with the addition of 50% excess bismuth. As is evident from these results, there is a good agreement between the results obtained and the expected composition. This is one of the major advantages of using the MOD process for depositing multi-component films. The final composition of these films is closely related to the stoichiometry and stoichiometry of the precursor. The starting materials used in this particular embodiment are sufficient to meet compositional control requirements.

[0044]

[Table 3]

X-ray diffraction analysis was performed to determine the phases formed under such deposition conditions. Figure 9 shows 400 ℃, 600 ℃,
3 shows diffraction patterns obtained from SBT films annealed at 650 ° C. and 750 ° C. for 3 hours. As can be seen from the XRD data, these films annealed at 400 ° C were amorphous. The formation of the crystalline phase started at a temperature close to 600 ° C and was completed at a temperature close to 750 ° C. As can be observed from this data, the film annealed at 750 ° C has (115),
(200), (00/10), (220), (20/10) and (31
5) It shows a strong diffraction peak of the plane. Annealing temperature is 7
In the case of 50 ° C., annealing for 1 hour gives a film similar to the film obtained by annealing for 3 hours.

Finally, the ferroelectric and degradation properties of the film determine its practical applicability. In this particular embodiment, the applicability of ferroelectric capacitors to non-volatile memory was tested. The hysteresis characteristics of the film with Pt electrodes were measured using a standard RT66A ferroelectric tester (Radian Technology). The upper electrode uses a shadow mask and has an area of 2.1 by RF sputtering.
It was deposited at × 10 ^-4 cm ² . FIG. 10 shows the hysteresis characteristic of the SBN film when a voltage of 5 V is applied. The hysteresis loop is fully saturated at this voltage,
A 2P _r value of C / cm ² was obtained. The fatigue test for these films was conducted at a frequency of 1MHz input from the pulse generator.
5V square wave ac signal. FIG. 11 shows 1
The result of the fatigue test performed up to ⁰⁹ cycles is shown. These films showed no fatigue until this cycle was performed, and the polarization loss was only 4% even after 10 ⁹ cycles, which is a negligible value. As shown in FIG. 12, this was confirmed by measuring the hysteresis characteristics of the film after the fatigue test. Hysteresis properties of these films are similar before and after fatigue testing. The leakage current density values for these films were measured as a function of applied voltage. When a voltage of 5 V was applied, these films showed a leakage current density of 2 × 10 ⁻⁸ A / cm ² .

These results clearly show that the layered structure oxides according to the invention are far superior to the materials prepared according to the prior art. The process of the present invention can provide a novel method capable of producing a layered structure oxide thin film having a ferroelectric property that is high enough to be applied to a device. The particular embodiments described above with reference to the drawings are for the purpose of illustration only, and should not be construed as limiting the invention described in the appended claims. For example, the process of the present invention for applying to a ferroelectric non-volatile random access memory,
It can also be used to produce thin films of high quality layered structure oxides. The process may also be applied to manufacture these materials in other applications for piezoelectric, pyroelectric, electro-optical devices, etc. This process is also applicable to deposit these materials on structures (which may be of different sizes) other than the (capacitors) specifically described in this invention. The process can also be modified to take additional steps. However, the basic concept of the present invention is the same in that case as well.

[0048]

The method for producing a ferroelectric thin film of the present invention provides a highly reliable chemical solution-based process for producing a ferroelectric thin film comprising a layered structure oxide containing no components that are easily vaporized at low temperatures. To do. Therefore, by using the ferroelectric thin film obtained by the manufacturing method of the present invention, it becomes possible to overcome the problems of deterioration such as fatigue, time-dependent dielectric breakdown and aging in the nonvolatile memory.

The ferroelectric thin film obtained by the manufacturing method of the present invention has excellent characteristics and reliability, and includes capacitors, nonvolatile memory elements, pyroelectric infrared sensors, optical display elements, optical switches, piezoelectric transducers, and It can be applied to various elements such as a surface acoustic wave element.

[Brief description of drawings]

FIG. 1 is a diagram showing a typical hysteresis loop of a ferroelectric material.

FIG. 2 is a schematic diagram showing a typical ferroelectric capacitor.

FIG. 3 is a schematic diagram showing a ferroelectric capacitor provided with a buffer layer.

FIG. 4 illustrates SrBi using a particular embodiment of the invention.
FIG. 3 is a schematic diagram showing a flowchart for preparing a ₂ (Ta _x Nb _2-x ) O ₉ (0 <x <2) film.

FIG. 5 is a diagram showing the refractive index and extinction coefficient of an SBT film as a function of wavelength.

FIG. 6 is a diagram showing the refractive index and extinction coefficient of an SBN film as a function of wavelength.

FIG. 7: S annealed on a Pt electrode at 750 ° C. for 3 hours
It is a figure which shows the microstructure of a BT film.

FIG. 8: S annealed on a Pt electrode at 750 ° C. for 3 hours
It is a figure which shows the microstructure of a BN film.

FIG. 9 shows the XRD pattern obtained on the SBT film as a function of annealing temperature.

FIG. 10: Pt / SBN / Pt before fatigue cycle
It is a figure which shows the hysteresis characteristic of a capacitor.

FIG. 11 is a diagram showing fatigue behavior of a Pt / SBN / Pt capacitor at an applied voltage of 5 V and a frequency of 1 MHz (bipolar square wave).

FIG. 12 Pt / SBN / P after fatigue cycle
It is a figure which shows the hysteresis characteristic of t capacitor.

[Explanation of symbols]

 10 Substrate 11 Intermediate Adhesive Layer 12 Lower Electrode Layer 13 Ferroelectric Layer 14 Upper Electrode

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] June 28, 1995

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] Figure 7

[Correction method] Change

[Correction content]

FIG. 7: S annealed on a Pt electrode at 750 ° C. for 3 hours
It is a SEM micrograph which shows the microstructure of a BT film.

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] Figure 8

[Correction method] Change

[Correction content]

FIG. 8: S annealed on a Pt electrode at 750 ° C. for 3 hours
3 is an SEM micrograph showing the microstructure of a BN film.

Claims (14)

[Claims] 1. A method of depositing a thin film of a ferroelectric layered structure oxide on a substrate by chemical solution deposition, the layered structure being in the form of an alkoxide, carboxylate, or acetylacetonate, or a combination thereof. A step of preparing a precursor solution containing a mixture of metal elements of oxides and a solvent; a step of applying the precursor solution on the substrate to form a thin film; And a step of removing the solvent from the film, and heating the substrate having the film to form a crystalline thin film layered structure oxide. 2. The ferroelectric layered structure oxide is at least one of the following compounds: A _n Bi ₃ Ti _{n + 1} RO _{3n + 9} ABi ₂ R ₂ O ₉ Bi _{2n + 2} Ti _{4- n} O _12-n A _{n + 1} Bi ₄ Ti _{n + 4} O _{15 + 3n} where A is Ca, Pb, Sr, or Ba, and R
Is Nb or Ta, and n = 0 or 1. The method of claim 1. 3. The method of claim 2, wherein the ratio of Ta to Nb is approximately 0.4. 4. The substrate is Si, SiO ₂ coated S
i, or a semiconductor composed of at least one of GaAs, sapphire, ZrO ₂ , MgO, SrTiO ₃ ,
The method according to claim 1, which is at least one of a single crystal insulator composed of at least one of BaTiO ₃ and PbTiO ₃ , and a complex integrated circuit. 5. The method of claim 1, wherein preparing the precursor comprises a hydrolysis process and a condensation process. 6. The method according to claim 1, wherein the solvent is at least one of alcohol, aromatic hydrocarbon and ester. 7. The method according to claim 1, wherein the precursor contains an excess of one or more metal elements of the layered structure oxide by 1% to 80%. 8. The method of claim 4, wherein the precursor is applied to the substrate using at least one of a spin coating process, a dipping process, a spraying process, or a coating process. 9. The method according to claim 1, wherein the calcination is performed at a temperature between 50 ° C. and 250 ° C. 10. The method of claim 1, wherein the heating is performed at a temperature between 500 ° C and 850 ° C. 11. The heating is carried out in air or in a controlled atmosphere of at least one of oxygen, nitrogen, argon or other inert gases.
The method described in. 12. The method of claim 1, wherein the fixed period of time is between 1 second and 20 hours. 13. The layered structure material is SrBi ₂ (Ta _x
Nb _2−x ) O ₉ (0 <x <2), and the precursor is strontium-2-ethylhexanoate (Sr (C ₇ H ₁₅ COO) ₂ , about 99.8%), A 50% excess of bismuth-2-ethylhexanoate (Bi (C ₇
H ₁₅ COO) ₃ , about 99%), tantalum ethoxide (Ta (OC ₂ H ₅ ) ₅ , about 99.95%), niobium ethoxide (Nb (OC ₂ H ₅ ) ₅ , about 99.95%) and 2-
Ethyl hexanoic acid (C ₇ H ₁₅ COOH, about 99%), the solvent is xylene, and the substrate is Si / S.
iO ₂ / Ti / Pt, the step of applying the precursor to the substrate is a spin coating method, the firing temperature is 250 ° C., the firing time is 3 minutes, and the heating is performed. The method according to claim 1, wherein the temperature is 750 ° C., and the heating time is 1 hour. 14. The ferroelectric thin film has a low fatigue rate,
The method of claim 13, wherein the amount of leakage current is low.